It has been known since approximately 1971 to use unstable resonator geometries to realize lasers which utilize a largest possible laser medium volume at the highest possible efficiency. In this connection, reference may be made to an article by A. E. Siegman entitled."Stabilizing Output with Unstable Resonators", Laser Focus, May 1971, pages 42 to 47.
The article of R. Beach et al entitled "Passively Q-switched Transverse-Diode-Pumped Nd.sup.3+ :YLF Laser Oscillator", Optics Letters, Volume 17, No. 2, pages 124 to 126, suggests that a flat output reflector be used in a resonator of this kind in order to obtain a mode selection, that is, to drive the laser, for example, only in a fundamental mode. The output reflector has a radially-dependent reflection profile. A passive Q-switch is provided in the resonator with an LiF-color center crystal. The above arrangement is not ideal if the most compact laser configuration with a high overall efficiency and simultaneous mode selectivity is to be realized. The reason for this is that a plurality of reflecting surfaces is present within the resonator chamber which causes different competing secondary resonators having so-called parasitic modes to-be formed. The quality of the beam is disadvantageously affected thereby.
A similar resonator configuration is also disclosed in U.S. Pat. No. 4,918,704. An unstable laser resonator is likewise disclosed there which has an output reflector having a radially-dependent profile. A Q-switch is also provided. A longitudinal mode selection takes place by means of a second diode laser which, however, is not absolutely necessary for specific applications. The plurality of reflecting surfaces present in the resonator facilitates also the formation of different competing secondary resonators during laser operation.
A suppression of competing secondary resonators is achieved in a conventional manner by beveling the end faces of the solid-state rod at the Brewster angle or arranging the output reflector of the resonator at an angle such as in the text of W. Kochner entitled "Solid State Laser Engineering", second edition, Springer Verlag, 1988, page 166. The modes formed in the corresponding resonators are, however, often not completely suppressed by means of these measures. Secondary foci occur when focusing the laser beam. Secondary foci of this kind can often only be completely eliminated with an extremely inclined position which, however, has a negative influence of the desired mode image.